SAN FRANCISCO, July 4 (Xinhua) -- A survey of the nutritional, physiological and reproductive health of southern resident killer whales, the smallest of four resident communities off the U.S. Pacific Northwest, suggests that up to two-thirds of pregnancies failed in this population from 2007 to 2014.
Published recently in the journal PLOS ONE, the study links these endangered whales' low reproductive success to stress brought on by low or variable abundance of their most nutrient-rich prey, Chinook salmon.
"Based on our analysis of whale health and pregnancy over this seven-year period, we believe that a low abundance of salmon is the primary factor for low reproductive success among southern resident killer whales," said lead author Sam Wasser, a University of Washington (UW) professor of biology and director of the Center for Conservation Biology.
"During years of low salmon abundance, we see hormonal signs that nutritional stress is setting in and more pregnancies fail, and this trend has become increasingly common in recent years."
Southern resident killer whales typically feed from May to October in the Salish Sea, an intricate network of coastal waterways that includes the southwestern portion of the Canadian province of British Columbia and the northwestern portion of the U.S. state of Washington, and spend winters in the open Pacific Ocean along the U.S. West Coast. Unlike transient orca populations that feed on marine mammals, more than 95 percent of the diet of southern resident orcas consists of salmon, with Chinook salmon alone making up about three-quarters of their total diet.
Researchers knew that the southern residents, just 78 individuals in December 2016, had a lower fecundity rate compared with orcas in northern British Columbia and southern Alaska. But the data gathered by the team researchers from UW and the U.S. National Oceanic and Atmospheric Administration (NOAA) indicate that dwindling and variable salmon runs do more direct damage to the reproductive success of the southern resident population than increasing boat traffic in the Salish Sea.
To gather data about orca health and reproduction, Wasser and his colleagues measured the breakdown products of key physiological and sex hormones in orca fecal samples, or scat, and used orca deoxyribonucleic acid (DNA) extracted from the scat to determine sex, family pod and identity of the individual responsible for the leavings. They collected 348 scat samples from 79 orcas between 2007 and 2014. On these fecal searches, the team also gathered extensive data on boat traffic in the area, which increased significantly during the study period.
The hormone levels they calculated from scat include progesterone, testosterone, glucocorticoid and thyroid hormone. As glucocorticoid and thyroid play key roles in physiological stress responses, determining levels of both hormones allowed researchers to differentiate between stress due to poor nutrition and stress due to external responses, such as boat traffic.
The researchers used progesterone and testosterone levels from females to determine reproductive state, and then correlated these data and the date of collection with calf sightings to determine whether each pregnancy was successful.
These hormone data detected 35 unique pregnancies among southern resident females from 2007 to 2014. In 11 cases, the individual female gave birth and was seen with a calf thereafter. But in 24 cases, or 69 percent of total pregnancies, no live calf was subsequently seen, indicating that these pregnancies failed.
In most cases, the pregnancies likely ended in spontaneous abortion during the first half of gestation. But in one-third of the failed pregnancies, hormone levels indicated that the calf was lost in the latter half of pregnancy or moments after birth. These females also showed signs of nutritional stress, with ratios of thyroid hormone relative to glucocorticoid hormone nearly seven times lower than females who successfully gave birth.
"These findings indicate that pregnancy failure, likely brought on by poor nutrition, is the major constraining force on population growth in southern resident killer whales," Wasser was quoted as saying in the news release from UW.
